diff --git a/src/agents/models/chatcmpl_converter.py b/src/agents/models/chatcmpl_converter.py
index 3a959fbef6..92207ca12c 100644
--- a/src/agents/models/chatcmpl_converter.py
+++ b/src/agents/models/chatcmpl_converter.py
@@ -763,7 +763,10 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
                 content_items = reasoning_item.get("content", [])
                 encrypted_content = reasoning_item.get("encrypted_content")
 
-                item_provider_data: dict[str, Any] = reasoning_item.get("provider_data", {})  # type: ignore[assignment]
+                raw_provider_data = reasoning_item.get("provider_data")
+                item_provider_data: dict[str, Any] = (
+                    raw_provider_data if isinstance(raw_provider_data, dict) else {}
+                )
                 item_model = item_provider_data.get("model", "")
                 should_replay = False
 
diff --git a/tests/models/test_chatcmpl_reasoning_provider_data_none.py b/tests/models/test_chatcmpl_reasoning_provider_data_none.py
new file mode 100644
index 0000000000..8782daafd0
--- /dev/null
+++ b/tests/models/test_chatcmpl_reasoning_provider_data_none.py
@@ -0,0 +1,97 @@
+"""Regression test for chatcmpl_converter handling of reasoning items
+with ``provider_data`` explicitly set to ``None``.
+
+JSON roundtripping (and some external producers) can store a reasoning item
+where ``provider_data`` is the literal ``None`` rather than missing or a dict.
+``Converter.items_to_messages`` previously assumed the field was always either
+absent or a dict and called ``.get("model", "")`` directly on it, which raised
+``AttributeError: 'NoneType' object has no attribute 'get'`` for these
+otherwise valid items.
+"""
+
+from __future__ import annotations
+
+import json
+from typing import Any, cast
+
+import pytest
+
+from agents.items import TResponseInputItem
+from agents.models.chatcmpl_converter import Converter
+
+
+def _reasoning_item_dict_with_provider_data_none() -> dict[str, Any]:
+    return {
+        "type": "reasoning",
+        "id": "__fake_id__",
+        "summary": [{"type": "summary_text", "text": "thinking"}],
+        "content": [{"type": "reasoning_text", "text": "step"}],
+        "encrypted_content": None,
+        "status": None,
+        "provider_data": None,
+    }
+
+
+def test_items_to_messages_handles_provider_data_none() -> None:
+    """Converter must not crash when a reasoning item has provider_data=None."""
+    items = [cast(TResponseInputItem, _reasoning_item_dict_with_provider_data_none())]
+
+    # The bug was a hard AttributeError raised at conversion time. The exact
+    # contents of the resulting messages are not the focus here — we only need
+    # to assert that conversion completes for both Claude and non-Claude
+    # targets and returns a list.
+    messages_claude = Converter.items_to_messages(
+        items,
+        model="claude-sonnet-4",
+        preserve_thinking_blocks=True,
+    )
+    assert isinstance(messages_claude, list)
+
+    messages_gpt = Converter.items_to_messages(
+        items,
+        model="gpt-4o",
+        preserve_thinking_blocks=False,
+    )
+    assert isinstance(messages_gpt, list)
+
+
+def test_items_to_messages_handles_provider_data_none_after_json_roundtrip() -> None:
+    """JSON serialization preserves a None value, exercising the same path."""
+    item_dict = _reasoning_item_dict_with_provider_data_none()
+    roundtripped: dict[str, Any] = json.loads(json.dumps(item_dict))
+
+    # Sanity check: the None survives the roundtrip.
+    assert roundtripped["provider_data"] is None
+
+    messages = Converter.items_to_messages(
+        [cast(TResponseInputItem, roundtripped)],
+        model="claude-sonnet-4",
+        preserve_thinking_blocks=True,
+    )
+    assert isinstance(messages, list)
+
+
+@pytest.mark.parametrize(
+    "bogus_provider_data",
+    [123, "not-a-dict", ["model", "x"]],
+)
+def test_items_to_messages_handles_non_dict_provider_data(
+    bogus_provider_data: object,
+) -> None:
+    """Non-dict provider_data values are treated as missing rather than crashing."""
+    item_dict: dict[str, Any] = {
+        "type": "reasoning",
+        "id": "__fake_id__",
+        "summary": [{"type": "summary_text", "text": "thinking"}],
+        "content": [{"type": "reasoning_text", "text": "step"}],
+        "encrypted_content": None,
+        "status": None,
+        "provider_data": bogus_provider_data,
+    }
+
+    messages = Converter.items_to_messages(
+        [cast(TResponseInputItem, item_dict)],
+        model="claude-sonnet-4",
+        preserve_thinking_blocks=True,
+    )
+    assert isinstance(messages, list)